Generally, for convenience and because standard semiconductor tools can be used in the fabrication process, X-ray masks are formed on a semiconductor wafer, such as a silicon wafer. The silicon wafer operates like a frame and support for the mask. A thin membrane is deposited on the upper surface of the wafer in the form of a thin layer. A layer of X-ray absorbing material, such as some safe heavy metal or alloy, is deposited on the upper surface of the membrane layer. The X-ray absorbing layer is patterned by applying a hard mask material and resist. The resist is patterned (or exposed) with an electron beam (E-beam) device and a hard mask is formed by etching the hard mask layer through the patterned photoresist layer. The hard mask is then used as an etch mask to pattern the X-ray absorbing layer. At some point in the process the wafer is etched from the membrane layer in a circle or a rectangle to form a thin membrane. The mask thus allows X-rays to pass through the thin membrane and portions of the X-ray absorbing layer that have been etched away. The entire procedure is known as a process flow and two different process flows are commonly used.
In the first process flow, commonly referred to as a wafer flow, all processing is done on the wafer with one of the final steps being the back etching of the silicon wafer to form the membrane. The wafer flow was primarily created to solve formatting issues. It allows X-ray mask processing to be conducted in semiconductor tools that are not dramatically different from the standard wafer processing tools supplied by the industry. The X-ray mask specific processing steps (membrane formation and wafer mounting to a support ring) are at the end of the flow. This minimizes the modifications necessary to both the tools and the wafer processes. However, the creation of the membrane and the mounting of the wafer creates significant pattern displacement errors (.about.0.15 .mu.m) in the mask.
The second process flow is commonly referred to as a membrane flow. In the membrane flow the membrane is formed early in the process (generally after absorber deposition) and the remaining processing is carried out on the membrane. The membrane flow process was derived to remove the errors in the wafer flow process by conducting the mask specific processing steps before the absorber layer is patterned. While this greatly reduces the errors associated with membrane formation and wafer mounting, it also increases the modifications to both the equipment (the tools must accept an X-ray mask format rather than a wafer) and the processes (the patterning defining process steps are carried out on a membrane rather than a wafer).
The major problem in the formation of X-ray masks is that the membrane layer generally is formed with tension stress and the absorbing layer develops stresses during the process or flow. These stresses are altered radically by the patterning of the absorbing layer and by the removal of the portion of the wafer required to form the membrane. The various stresses produce distortions in the pattern, which distortions are, or result in, displacement of portions of the pattern. To offset these distortions the pattern written on the absorber is predistorted in an attempt to cancel the displacements. The problem is that while some of the major distortions have been offset, many residue or minor errors are not offset and, thus, greatly limit the accuracy and usefulness of X-ray masks.
It is a purpose of the present invention to provide new and improved methods of fabricating X-ray masks.
It is another purpose of the present invention to provide new and improved methods of fabricating X-ray masks which compensate for residual errors so as to greatly improve the accuracy.
It is a further purpose of the present invention to provide new and improved methods of fabricating X-ray masks including the use of a new function which can accurately describe the movement expected from all parts of a rectangular membrane in a simple closed form.